The present invention relates to a bistable actuation device. As known, an actuation device of the above mentioned type is able, during a first operation cycle, to reach a determined work condition and maintain it and, then go back to its previous work condition following a subsequent operation cycle. Such actuation devices, which have actuation means as well as means for maintaining the conditions reached at the end of the first operation cycle, are opposed to the monostable devices or actuation systems, which are not equipped with means for maintaining the work position reached at the time the electric supply is interrupted.
Bistable actuation devices have a plurality of applications in practice; for instance, they are employed to manufacture door-lock systems for domestic appliances, such as washing-machines or cooking apparatuses, in order to avoid dangerous operations for users.
Bistable actuation devices are known, wherein the condition reached is maintained by means of a pair of bimetallic elements; however, such systems have no adequate means for functionality control, so that their operation, which is based on the surrounding temperature, can be easily affected by external heat sources. Electromagnetic bistable actuation systems are also known, which require the use of complex circuits for controlling either their polarity inversion or electric switching.
Another typical problem according to the state of the art arises from the complexity of the components used in bistable devices and their arrangement; for instance, the use of electromagnets makes such actuation devices complicated, bulky and expensive.
A further problem is also due to the fact that many known actuation devices, having a small size, are not able to develop a considerable force and/or have no kinematic motion assemblies able to oppose high forces when the actuator is no longer electrically supplied.
From U.S. Pat. No. 5,656,989 a device is also known, consisting of a monostable actuator, in particular of the electro-thermal type, having kinematic means for maintaining a work position when there is no electric supply; such means comprise at least a housing body wherein there are arranged in series:
a first element, which slides linearly in the body under the thrust produced by the actuator; PA1 a second element, which slides and rotates inside the body under the thrust produced by the first element, with which an actuating shaft is associated; PA1 a third element, which slides linearly within the body under the thrust produced by the second rotatable element and is acted upon by a spring.
The first two elements have slanting toothed surfaces, which convert the linear motion of the first element into a linear and rotary motion of the second element; the second element has then means able to engage with the ends of guides being provided for the movement of the first element, which are obtained on an inner surface of the body and which allow the maintaining of a position for the second and third elements.
According to the device disclosed in U.S. Pat. No. 5,656,989, a linear motion produced by the actuator has to be transferred to an angular motion of a part of the thrust kinematic motion, in order to reach and maintain a stable work condition. Such an at least partial conversion of the motion exerted by the actuator, i.e. from linear to angular or rotary, may entail slight losses in the actuation force.
Moreover, the solution described in the above document, though being sophisticated and functional, requires accurate manufacture of the various elements, which results in a device cost sometimes not justified by its application. The above solution also requires good lubrication of the various components to warrant optimal operation.
A further constraint of U.S. Pat. No. 5,656,989 is that the described device necessarily requires a closed housing for the kinematic motion assembly which should have a certain length for containing all three elements and an internal circular section, and consequently high space requirements.
Moreover, the device disclosed in U.S. Pat. No. 5,656,989 has the drawback of not being easily manually resettable from outside, for instance with a proper mechanical tool if required (such as in the event of a blackout, a fault of the monostable actuator, etc.); such a drawback may be particularly problematic should the device be used in door lock systems of domestic appliances.